It’s a long story.
Definition 1 of 2

Casual Expression
Used informally among friends or in relaxed environments, not in formal situations.

You moved three times this year? It’s a long story.
Avoiding Details
The phrase is a polite way to avoid giving a lengthy explanation.

Why'd you quit your job? It’s a long story, let’s talk about something else.
Inviting Curiosity
It can spark listeners' curiosity and lead to them asking for more details.

She raised an eyebrow. 'It's a long story,' he said, pausing for effect.
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